The present invention relates to a method and apparatus for detecting scratches on the surface of a semiconductor wafer. In particular, the invention relates to detecting surface scratches on high quality, transparent, single crystal silicon carbide wafers that are polished on both sides.
Silicon carbide has found use as semiconductor material for various electronic devices and purposes in recent years. Silicon carbide is especially useful due to its physical strength and high resistance to chemical attack. Silicon carbide also has excellent electronic properties, including radiation hardness, high breakdown field, a relatively wide band gap, high saturated electron drift velocity, and high-temperature stability.
Single crystal silicon carbide is often produced by a seeded sublimation growth process. In a typical silicon carbide growth technique, the seed crystal and a source powder are both placed in a reaction crucible which is heated to the sublimation temperature of the source and in a manner that produces a thermal gradient between the source and the marginally cooler seed crystal. The thermal gradient encourages vapor phase movement of the materials from the source to the seed followed by condensation upon the seed and the resulting bulk crystal growth. The method is also referred to as physical vapor transport (PVT).
The bulk single crystal of silicon carbide may then be desirably cut into wafers and polished prior to the growth of epitaxial layers and the formation of devices on the wafers. Common techniques for polishing silicon carbide wafers sometimes scratch the wafer surface. The location, size, and depth of these scratches are best identified early before they become problematic. The quality of the growth surface affects the quality of any deposited epitaxial layers, which in turn affects the quality of any resulting devices formed in or from the SiC and the epilayers.
In some conventional techniques, a skilled person visually inspects a wafer to locate scratches, attempting to identify scratches by holding the wafer up to the light. The inspection identifies the side of the wafer containing scratches allowing quality control for the side used for semiconductor device preparation. This technique suffers several drawbacks, including frequent inaccuracy, subjectivity, and potentially the need for persons to touch the wafer surfaces.
Other scratch detection techniques offer more precision by directing light at the wafer surface, and then measuring (using some sort of detector) the light as reflected or scattered from the wafer surface to identify various flaws.
High quality single crystal silicon carbide is often (and intentionally) transparent in the visible frequencies, as well as some of the UV frequencies. Thus, light in the visible wavelengths fails to discriminate between flaws (scratches) on the growth surface and flaws on the opposite surface. This problem is exacerbated in “double-side polished” wafers; i.e. those that have been sliced from boules and polished on both faces.
If the scratch remains undetected until after production of a device on the wafer surface, then costly reworks and repolishing steps must be undertaken to correct this mistake. Such reworking does not, however, address the detection problem and thus does not guarantee that the same mistake will not be repeated.
Efficient techniques for accurately identifying the presence of scratches on a semiconductor wafer surface prior to device preparation are therefore needed, including techniques capable of successfully identifying scratches on transparent double-side polished wafers.